Final answer:
The distributive property of multiplication over subtraction leads to 3(x - 1) being equal to 3x - 3.
Step-by-step explanation:
The distributive property in mathematics allows you to multiply a single term by each of the terms inside a set of parentheses. Applying this property to 3(x - 1) means you multiply 3 by x and also multiply 3 by -1. Therefore, when you distribute 3 over the subtraction inside the parentheses, you get 3x - 3. So, the correct answer to fill in the blank is '3', which corresponds to answer choice (d).